Non-reciprocal circuit element

ABSTRACT

The invention relates to a non-reciprocal circuit element ( 1 ) having a plurality of strip conductor elements ( 2 ) insulated electrically from one another, which conductor elements are embedded in a multilayer core ( 3 ) of ferrimagnetic material and are arranged in superposed conductor planes in such a way that the conductor elements ( 2 ) cross over one another in at least one crossover area ( 4, 5 ). To provide such a circuit element, which is particularly cost-effective to produce and which is suitable in particular for use in mobile phones, the invention proposes that the core ( 3 ) comprises, at least in the crossover area of the conductor elements ( 2 ), hard magnetic material, which is permanently magnetized in a spatial direction perpendicular to the conductor planes.

The invention relates to a non-reciprocal circuit element having aplurality of strip conductor elements insulated electrically from oneanother, which conductor elements are embedded in a multilayer core offerrimagnetic material and are arranged in superposed conductor planesin such a way that the conductor elements cross over one another in atleast one crossover area.

Such non-reciprocal circuit elements comprise circulators or isolators,for example. These are used inter alia in mobile phones, where they areconnected between the output of the booster and the antenna. Thenon-reciprocal circuit element is intended to protect the output of thebooster from radio frequency signals reflected at the antenna. In thecase of a mismatched mobile phone antenna, some of the radio frequencysignals output by the booster are reflected, such that the output of thebooster is loaded with radio-frequency signals of considerable power.Antenna mismatches arise virtually constantly with conventional mobilephones, since the impedance of the narrow band antennae used is stronglydependent on environmental influences. The radio-frequency powerreflected onto the booster disadvantageously produces distortions in thesignals emitted by the mobile phone. Such signal distortions areundesirable, especially in so-called third generation mobile phones,since a linear and thus distortion-free transmission characteristic isabsolutely essential for error-free functioning of the modulation anddemodulation technology used in these devices.

A non-reciprocal circuit element of the above-mentioned type is known,for example, from EP 0 618 636 B1. This publication relates to acirculator, in which the strip conductor elements insulated electricallyfrom one another are embedded in a core of soft magnetic ferrite. Thecore consists of a plurality of superposed layers of YIG (yttrium irongarnet), which are sintered together during production of the previouslyknown circulator. In order that the gyromagnetic effect required for thecirculator to function occurs, the soft magnetic material of the corehas to be magnetized in the case of the previously known circulator bytwo permanent magnets arranged above and below the core. The entirearrangement is surrounded by a metallic housing, which serves as amagnet yoke.

The primary disadvantage of the previously known circulator is that theproduction thereof is associated with high production costs, inparticular because the positioning of the permanent magnets on the coreof the previously known circulator has to be extremely precise, with thesmallest possible mechanical tolerances, as does assembly of the housingserving as a magnet yoke. The magnetization of the core and thus itsgyromagnetic behavior are greatly influenced by the positioning of thepermanent magnets. Even slight tolerances in assembly of the previouslyknown circulator may therefore have a catastrophic effect on theelectrical characteristics thereof. This may result in a need forsubsequent tuning and adjustment of the circuit element duringproduction, which further increases production costs. A furtherdisadvantage of the previously known circuit element is its relativelylarge size, which is determined primarily by the large amount of spacerequired by the permanent magnets.

In so-called third generation mobile phones, the use of non-reciprocalcircuit elements is absolutely essential for the reasons outlined above.Because of the large numbers of such circuit elements required in themobile phone sector, it is desirable to be able to manufacture them atthe lowest possible cost. Since modern mobile phones have to becompatible with a plurality of transmission standards (e.g. GSM, UMTSetc.) and since it is necessary for this purpose to incorporate a largenumber of separate circuit elements for the respective frequency bandsin one device, the dimensions of the individual circuit elements have tobe smallest possible.

Accordingly, it is an object of the present invention to provide afurther-developed non-reciprocal circuit element which has particularlysmall dimensions and may be produced at low cost.

Taking a non-reciprocal circuit element of the above-mentioned type asbasis, this object is achieved in that the core comprises, at least inthe area where the conductor elements cross over one another, hardmagnetic material, which is permanently magnetized in a spatialdirection perpendicular to the conductor planes.

In contrast to the soft magnetic materials used in conventionalnon-reciprocal circuit elements, the hard magnetic material usedaccording to the invention for the core has a strong remanentmagnetization, which means that the core may be magnetized on a one-offbasis during production, such that the finished circuit element managescompletely without permanent magnets. Manufacturing tolerances are ofvirtually no significance, since the magnetic field acting on thecircuit element for magnetization may be adjusted so as to correspond tothe desired specification of the circuit element.

Because, according to the invention, the fitting of permanent magnets tothe non-reciprocal circuit element is unnecessary and because mechanicaltolerances are thereby of virtually no significance in assembly of thecircuit element, a considerable reduction in production costs relativeto the prior art is achieved. Furthermore, the spatial dimensions of thecircuit element according to the invention are markedly reduced relativeto the circuit elements known from the prior art because of the lack ofpermanent magnets. It is clear that the circuit element according to theinvention, whose electromagnetically active core comprises hard magneticmaterial, is well suited to third generation mobile phone applications.Barium hexaferrite is an example of a suitable material for the core.

In an appropriate further development of the invention, thenon-reciprocal circuit element comprises an upper and a lower outerlayer of soft magnetic material. After magnetization of the core, themagnetization in the outer layers is so aligned that a closed-loopmagnetic field pattern is automatically established. The soft magneticouter layers function to a certain extent as a magnet yoke.

It is particularly appropriate for the upper and/or lower outer layersto be separated from the core each by an electrically conductiveseparator layer. This electrically conductive separator layer shouldadvantageously be grounded. In this way it is ensured that theelectromagnetic radio-frequency signals propagate solely in the hardmagnetic core of the circuit element and do not penetrate for instanceinto the soft magnetic layer, thereby reducing signal losses.

The strip conductor elements of the non-reciprocal circuit elementaccording to the invention should advantageously cross over one anotherin pairs at an angle of 120°. Three conductor elements arrangedaccordingly produce a circulator with three terminals.

In a particularly advantageous further development of the circuitelement according to the invention, two spatially separate crossoverareas of the conductor elements are provided, the hard magnetic materialof the core being oppositely magnetized in the respective crossoverareas. In this way, a circulator with four terminals may be particularlysimply produced, which comprises two circulators with three terminals,one of the conductor elements simultaneously forming the output of theone and the input of the other circulator. If the circuit element isconstructed according to the invention in three layers, the hardmagnetic core being surrounded by an upper and a lower outer layer ofsoft magnetic material, the opposite magnetization of the coreadvantageously produces, as it were automatically, a closed-loop fieldpattern within the component. Metallic housing parts serving forinstance as a magnet yoke are unnecessary in the circuit elementproduced accordingly, which in turn leads to low production costs and toa reduction in the dimensions of the circuit element.

Non-reciprocal circuit elements according to the invention mayadvantageously be produced from ceramic substrates in conventionalmultilayer technology. HTCC and LTCC (high/low temperature cofiredceramic) technologies are likewise possible. Such production processesusually begin with cutting “green” foils of unfired ceramic substrate tosize. Plated-through openings are then produced in these foils, whichopenings are filled with electrically conductive conductor paste. Thestrip conductor elements required for the non-reciprocal circuit elementare then printed onto the foils, for example by screen printing orstencil printing. Once the foils have been dried, they are stacked intoa foil stack, which is then compacted and subsequently sintered in afurnace. When producing a non-reciprocal circuit element according tothe invention, the foil stack comprises a plurality of inner foils ofhard magnetic material and at least one upper and at least one lowerouter foil of soft magnetic material, the strip conductor elements beingprinted on the inner foils in such a way that conductor elementssuperposed in the foil stack cross over one another in at least onecrossover area. Electrically conductive separator layers between theouter foils and the inner foils may be produced by metallizing theentire surface of the corresponding outer and inner foils respectively.A final method step in the production of the non-reciprocal circuitelement according to the invention comprises magnetization of thesintered foil stack in a direction perpendicular to the foil planes. Inthis way, the hard magnetic material of the core is permanentlymagnetized in accordance with the specification of the circuit element.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted. In the Figures:

FIG. 1 is an exploded view of a 4 port circulator according to theinvention;

FIG. 2 is a plan view of the circulator according to FIG. 1;

FIG. 3 is a cross-sectional representation of the circulator.

The 4-port circulator 1 illustrated in the Figures comprises a pluralityof strip conductor elements 2 electrically insulated from one another.As is clear from FIG. 3, these are embedded in a core 3, whichcomprises, according to the invention, hard magnetic material, forexample barium hexaferrite. The conductor elements 2 are arranged inmutually superposed conductor planes and cross over one another in twocrossover areas 4 and 5. The arrows 6 in FIG. 3 indicate that the hardmagnetic material of the core 3 is permanently magnetized in a spatialdirection perpendicular to the conductor planes. The circulatorillustrated comprises an upper outer layer 7 and a lower outer layer 8of soft magnetic material. The material may be YIG (yttrium irongarnet), for example. As shown by the symbols 9 in FIG. 2 and the arrows6 in FIG. 3, the hard magnetic material of the core 3 is oppositelymagnetized in the respective crossover areas 4 and 5. The arrows 10illustrated in FIG. 3 show that the magnetization in the soft magneticmaterial of the upper and lower outer layers is so aligned that aclosed-loop field pattern is produced. The magnetic field lines insidethe circulator 1 then exhibit a closed-loop pattern. The oppositelymagnetized areas of the core 3 are separated from one another in FIGS. 2and 3 by a broken line 11. The 4-port circulator illustrated in theFigures comprises in principle two 3-port circulators, which areconnected together by means of the conductor element 2 extendinghorizontally in FIG. 2. A crossover area 4 or 5 is assigned to each ofthe two 3-port circulators, respectively. In FIG. 2, the four signalterminals of the circulator carry reference numeral 12. Terminals 13serve to ground the circuit element. FIG. 3 shows two electricallyconductive layers 14, by means of which the upper and lower outer layers7 and 8 respectively are separated from the core 3.

FIG. 1 clearly shows the multilayer structure of the circulatoraccording to the invention. The core 3 comprising hard magnetic materialis composed of a total of seven layers. The strip conductor elements 2are arranged on the three middle layers in such a way that therespective conductor planes come to lie over one another, resulting inthe crossover pattern illustrated in FIG. 2. The conductor elements 2cross over one another in pairs at an angle of 120°. According to FIG.1, the upper outer layer 7 is composed of two layers of soft magneticmaterial. Likewise, the lower outer layer 8 comprises two soft magneticlayers, of which the upper one is metallized over its entire surface, soproducing the electrically conductive separator layer 14 for separatingthe core 3 from the lower outer layer 8. Moreover, the top hard magneticlayer of the core 3 is metallized over its entire surface, thus forminga second electrically conductive separator layer 14 for separating thecore 3 from the upper outer layer 7. Some of the layers of the circuitelement illustrated in FIG. 1 are provided with plated-through openings15 for contacting the conductor elements 2. FIG. 1 shows the structureof the foil stack into which the foils of unfired “green” ceramicsubstrate are stacked in the production method according to theinvention after they have been cut to size and provided withplated-through openings 15 and after the strip conductor elements 2 havebeen printed on, for example by means of screen or stencil printing.Once the illustrated foils have been stacked, the foil stack iscompacted and then sintered to yield the finished, non-reciprocalcircuit element 1. After the sintering process, the core 3 is magnetizedin accordance with the diagram illustrated in FIG. 3 by the applicationof appropriate external magnetic fields. Once these magnetic fields havebeen turned off, magnetization is established independently in the softmagnetic outer layers 7 and 8, said magnetization being indicated by thearrows 10 according to FIG. 3.

1. A non-reciprocal circuit element having a plurality of strip conductor elements (2) insulated electrically from one another, which conductor elements are embedded in a multilayer core (3) of ferrimagnetic material and are arranged in superposed conductor planes in such a way that the conductor elements (2) cross over one another in at least one crossover area (4, 5), characterized in that the core (3) comprises, at least in the area (4, 5) where the conductor elements (2) cross over one another, hard magnetic material, which is permanently magnetized in a spatial direction perpendicular to the conductor planes.
 2. A non-reciprocal circuit element as claimed in claim 1, characterized by an upper and a lower outer layer (7, 8) of soft magnetic material.
 3. A non-reciprocal circuit element as claimed in claim 2, characterized in that the upper and/or lower outer layers (7, 8) are separated from the core (3) each by an electrically conductive separator layer (14).
 4. A non-reciprocal circuit element as claimed in claim 1, characterized in that the conductor elements (2) cross over one another in pairs at an angle of 120°.
 5. A non-reciprocal circuit element as claimed in claim 1, characterized by two spatially separate crossover areas (4, 5) of the conductor elements (2), the hard magnetic material of the core (3) being oppositely magnetized in the respective crossover areas (4, 5).
 6. A method for producing a non-reciprocal circuit element (1), having the method steps of: a) cutting foils of unfired ceramic substrate to size, b) producing plated-through openings (15) in the foils, c) filling the plated-through openings (15) with conductor paste, d) printing strip conductor elements (2) on the foils, e) drying the foils, f) stacking the foils into a foil stack, g) compacting the foil stack, h) sintering the foil stack, characterized in that the foil stack comprises a plurality of inner foils (3) of hard magnetic material and at least one upper and at least one lower outer foil (7) of soft magnetic material, the strip conductor elements (2) being printed on the inner foils (3) in method step d) in such a way that conductor elements (2) superposed in the foil stack cross over one another in at least one crossover area (4, 5).
 7. A method as claimed in claim 5, characterized in that, in the foil stack, the outer foils (7, 8) are separated from the inner foils (3) by in each case an electrically conductive separator layer (14).
 8. A method as claimed in claim 5, characterized in that the sintered foil stack is magnetized in a method step i) in a direction perpendicular to the foil planes. 